Various systems exist for determining the angular position of a rotating member. In some known systems, a permanent magnet is mounted to or embedded in an end of a rotatable shaft and is centered on the axis of rotation. A magnetic field sensor, such as a Giant Magnetoresistive sensor (GMR), Anisotropic Magnetoresistance (AMR) sensor, Hall-sensor, etc. is positioned so as to sense changes in the magnetic field generated by the magnet in response to its change in angular position as it rotates with the shaft. The resulting output signals generated by the magnetic sensor are indicative of the angular position of the rotating shaft. Some known systems employ a permanent magnet ring attached to the perimeter of the shaft, and the direction of field lines above or beside the magnet is detected by the magnetic field sensor.
However, the magnetic field lines are not straight, which results in an angle error defined as the difference between the magnetic field direction detected by the sensor versus the rotation angle of the shaft. This field error depends on the shape of the magnet and the location of the sensor. The sensor's reading radius is the distance from the sensor to the axis of rotation. At some distances, most of the magnetic field lines cross the sensor at the same angle, and hence, the sensor would see little or no angle error at such a distance. However, in practice magnetic field sensors have position tolerances in the order of 0.1 mm, mainly due to manufacturing inaccuracies during die attach. Therefore the sensors can be misplaced slightly, which in turn can result in angle errors.
Further, known sensors such as the type described above can be sensitive to external magnetic fields. For instance, if a motor is being operated nearby, the magnetic field produced by the motor can be superimposed on the field produced by the magnet attached to the rotating shaft, resulting in angle errors.